It may be due to a relatively thin toolbar I'm using, which leaves a gap between previews when hovering and the toolbar button the mouse pointer is hovering above. In case of grouped display of previews, moving mouse pointer to one of the previews is needed - but this is hard to do when the mouse pointer must first cross the gap between tool bar and preview: with short delay time (minimum of 5ms), the previews close before the mouse pointer reaches them.
Workaround is, of course, to set hide delay somewhat longer - 100 ms is adequate - but 100 ms feels like half an infinity when used to snappy screen responses. A better solution may be to find a way to position the previews such that no gap between tool bar and previews show, IMHO.